In recent years, two-dimensional NMR spectroscopies have attracted interest as new NMR spectroscopies. They are two-dimensional correlation spectroscopy (COSY) in which a measurement is made in the sequence shown in FIG. 1(a), spin-echo correlation spectroscopy (SECSY) in which a measurement is made in the sequence shown in FIG. 1(b), two-dimensional correlation spectroscopy (NOESY) which makes use of the nuclear Overhauser effect (NOE) and in which a measurement is made in the sequence shown in FIG. 1(c), etc. In this way, various two-dimensional NMR spectroscopies are utilized, depending on the kind of information to be derived. These are able to extract useful information with high resolution.
Referring to FIGS. 1(a), (b), and (c), a free induction decay signal is detected for a period of time t.sub.2 and stored in a memory. This single measurement is repeated with successively different values of evolution period t.sub.1. Free induction decay signals are obtained by these measurements and stored in the memory, corresponding to the values of t.sub.1. The resultant set S (t.sub.1, t.sub.2) of the stored data is subjected to double Fourier transformation with respect to t.sub.2 and t.sub.1. In this way, a two-dimensional spectrum is provided. In FIG. 1(c), .tau. denotes a mixing period.
This two-dimensional NMR spectroscopy requires that measurement be repeated with several hundred to several thousand different values of t.sub.1.Further, an integration operation is performed for improving the signal-to-noise ratio in the conventional manner. Hence, it is inevitable that the measurements take as much as several to tens of hours. Therefore, if a plurality of two-dimensional spectra, e.g., a COSY spectrum, an SECSY spectrum, and an NOESY spectrum, are obtained from a single sample, the required time will triple. If the sample is stable, no problem will take place however long it takes. However, if the sample is unstable because it is a living body, for example, the sample will decompose before the three measurements are completed. This introduces the possibility that the obtained three spectra do not give the same information.
In view of the foregoing, C. A. G. Haasnoot et al. (Journal of Magnetic Resonance, 56 pp. 343-349, 1984) and A. Z. Gurevich et al. (Journal of Magnetic Resonance, 56, pp. 471-478, 1984) attempted to halve the measurement time compared with the case in which a COSY spectrum and an NOESY spectrum are separately obtained, by deriving the two spectra simultaneously by one measurement. More specifically, they have noted that the mixing period .tau. in the sequence for obtaining an NOESY spectrum as shown in FIG. 1(c) corresponds to the period during which a detection is made in the sequence for obtaining a COSY spectrum as shown in FIG. 1(a). This period is referred to as the second detection period t.sub.2 ', and during which a free induction decay signal is detected to give rise to a COSY spectrum. The sequence in which measurements are made is illustrated in FIG. 2. Free induction decay signals FIDa and FIDb which are obtained during the detection periods t.sub.2 ' and t.sub.2, respectively, are stored in their respective data files. COSY and NOESY spectra are produced from the free induction decay signals FIDa and FIDb, respectively.
Unfortunately, two-dimensional spectra obtained in accordance with these procedures are limited to COSY and NOESY spectra. When other kinds of two-dimensional spectra are to be obtained, additional measurements must be made.